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Fig. 2: Artist's views of GLAST (upper panel) and
VERITAS (lower panel) two next-generation
gamma-ray telescopes. (Images courtesy of GLAST and VERITAS collaborations)
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Fig. 3: This figure shows the detectability of various supersymmetric models.
Each point corresponds to a valid model resulting in a specific mass and corresponding
interaction cross-section of the neutralino particle. The lines show the detectability
limits for typical observations of GLAST and VERITAS. Points lying above the lines of
the telescopes indicate models that could be detected.
The lowest solid line corresponds to an observation of the galactic centre with GLAST,
the long dashed line to an observation of a Milky Way satellite galaxy like the Large
Magellanic Cloud (LMC). Whereas there are many models that could be detectable with GLAST
observations, very few models would produce enough gamma-rays to allow a detection
with VERITAS.
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In 1933 the swiss astronomer Fritz Zwicky observed the
velocities of galaxies in clusters of galaxies and found
a surprising result: The mass of the galaxies he saw was
far too low to explain their motions within the cluster.
He concluded that there had to be additional "dark" matter
associated with the galaxy clusters.
Today we know that about 90% of the of the total matter in
the Universe is not only dark - i.e. does not emit any
light - but in addition it must be made of some mysterious
yet unknown kind of particles. It is one of the greatest challenges
in cosmology today to identify the nature of this dark matter.
One of the most probable candidates for the dark matter is a
particle called neutralino. This particle arises naturally
in theories that extend the standard model of particle physics.
These supersymmetric theories introduce a new symmetry
- supersymmetry - which assigns to each boson a new
corresponding supersymmetric fermion particle and vice versa.
So far none of these newly predicted particles has been detected.
They are supposed to have energies too high to be probed with
current particle accelerators.
The neutralinos might, however, self-annihilate when they collide in
dense regions of the Universe, producing, among other particles, gamma
rays of high energy. The idea is to try to detect this radiation and
thus finally find out about the nature of the dark matter particle and its mass.
Dark matter annihilation is very sensitive to the density of the dark
matter, and so to the detailed structure of the dark halos that surround
our own and other galaxies. Our Milky Way is the prime target for detection,
especially its centre which is "only" about 26,000 lightyears away.
The MPA group used a large supercomputer at the Max Planck Society's
Garching Supercomputer Centre to simulate the assembly of a DM halo
very similar to our own with a world-record spatial resolution (Fig 1).
For different parameters of the supersymmetric theory they computed
the amount of gamma-radiation expected and compared it to the
detection limits of two next-generation gamma-ray telescopes -
one being a satellite mission (Fig 2. upper panel:
The Gamma Ray Large Area Space Telescope
GLAST) the other a ground based
telescope (Fig 2. lower panel Very Energetic Radiation Imaging Telescope
Array System VERITAS).
They found that with a new proposed detection strategy which searches
for gamma-rays over a wide area of the sky ten or twenty degrees away
from the Galactic Centre, there is a good chance that GLAST will
detect annihilation radiation from the inner regions of the Milky Way
(Fig 3). We might finally be able to "see" the dark matter and
unveil its still mysterious nature.
Felix Stoehr, Simon D. M. White, Volker Springel and Giuseppe Tormen
Further reading:
F. Stoehr, S. D. M. White, V. Springel and G. Tormen
"Dark Matter Annihilation in the Halo of the Milky Way",
MNRAS, volume 345, page 1313
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Fig. 1: Dark matter density distribution of the simulated Milky Way halo in
logarithmic scale. The image shown is 600,000 light years on a side. If future experiments
detect annihilation radiation it will likely be from inner Galaxy, a region a few percent
of the size of the one shown here.
